Ubiquitination of postsynaptic proteins is critical for learning and memory (Ehlers, 2003) and has been implicated in remodeling of synaptic networks in brain development, synaptic function, and brain disorders (Mabb and Ehlers, 2010). Ubiquitination is a regulated post-translational modification that conjugates ubiquitin moieties to lysine residues of target proteins via the sequential actions of an ubiquitin-activating enzyme (E1), an ubiquitin-conjugating enzyme (E2), and an ubiquitin ligase (E3) (Pickart, 2001). While polyubiquitination functions in targeting proteins for degradation by the proteasome, monoubiquitination functions in non-proteasomal signaling, including endocytosis and vesicular trafficking (Mukhopadhyay and Riezman, 2007). While Colledge et al. (2003) published that PSD-95 is ubiquitinated and rapidly removed from synaptic sites by proteasome-dependent degradation, ubiquitination independent of proteasome function has been implicated in N-Methyl-D-aspartic acid (NMDA) receptor (NMDAR)-induced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA) receptor (AMPAR) endocytosis and long-term depression (LTD) (Citri et al., 2009).
Postsynaptic density protein 95 (PSD-95) (synapse-associated protein 90(SAP90)) is a major postsynaptic scaffolding protein of glutamatergic synapses implicated in synaptic maturation and regulation of synaptic strength and plasticity (El-Husseini et al., 2000; Kim & Sheng, 2004; Funke et al., 2005; Elias & Nicoll, 2007). PSD-95 associates with AMPARs via its interaction with transmembrane AMPAR regulatory proteins (TARPs) (Chen et al., 2000; Schnell et al., 2002) and determines the synaptic density of AMPARs (Bats et al., 2007; Elias & Nicoll, 2007; Payne, 2008). Overexpression of PSD-95 enhances the surface expression of AMPARs (E1-Husseini et al., 2000; Schnell et al., 2002; Beique & Andrade, 2003), occludes long-term potentiation (LTP) (Stein et al., 2003; Ehrlich & Malinow, 2004), and enhances LTD (Stein et al., 2003). Knocking out (or down) PSD-95 and its family members decreases the synaptic levels of AMPARs (Beique et al., 2006; Elias et al., 2006), and PSD-95 knockdown inhibits NMDAR-induced endocytosis of synaptic AMPARs (Bhattacharyya et al., 2009) and impairs LTD (Xu et al., 2008).
PSD-95 is ubiquitinated by the E3 Ubiquitin Ligase murine double minute (Mdm)2 (also known as human double minute (Hdm)2), and the rapid and transient ubiquitination of PSD-95 has been implicated in NMDAR-induced AMPAR endocytosis by a mechanism involving proteasome degradation of ubiquitinated PSD-95 (Colledge et al., 2003). Since ubiquitination of PSD-95 is not easily detectable (Ehlers, 2003; Bingol & Schuman, 2004) and occurs transiently (Colledge et al., 2003; Mabb and Ehlers, 2010), it has not been clear whether PSD-95 is polyubiquitinated or monoubiquitinated (Colledge et al., 2003). Ubiquitination of PSD-95 requires the PEST motif (Rechsteiner and Rogers, 1996) within the N-terminus of PSD-95 (Colledge et al., 2003), proximal to the residues phosphorylated by cyclin-dependent kinase (Cdk)5 (Morabito et al., 2004). Cdk5 is a proline-directed serine/threonine kinase inactivated by NMDAR stimulation (Wei et al., 2005) that is implicated in synaptic plasticity, learning and memory, and in many disorders including drug addiction and neurodegenerative diseases such as Alzheimer's (Ohshima et al., 2005; Angelo et al., 2006; Cheung et al., 2006; Hawasli and Bibb, 2007; Lai 3 and Ip, 2009). The importance of Cdk5 in synaptic plasticity is underscored by the enhancement of LTP in conditional Cdk5 knockout mice (Hawasli et al., 2007) and the lower threshold for LTP induction (Wei et al., 2005) and impaired LTD (Ohshima et al., 2005) in mice with a deletion for the Cdk5 regulatory subunit p35.